This invention relates to an improved method for the preparation of a ureaform dispersion fertilizer, and the product resulting therefrom. More particularly it relates to a new method for preparing ureaform dispersion fertilizers from urea and monomethylolurea which are stable in soluble fertilizer salt solutions.
Liquid fertilizers are usually aqueous solutions of salts of ammonium, potassium, phosphate, and nitrate ions which frequently contain dissolved urea. In recent years liquid fertilizers have been developed which contain dispersed insoluble ureaform particles to lengthen the period of time over which the nitrogen content is released as a plant nutrient.
Ureaform is a term which is used in the fertilizer art to denote mixtures of urea-formaldehyde polymers. The overall water solubility of these polymers is low thereby precluding its releasing nitrogen at a rate high enough to burn or damage vegetation. Ureaform polymers may be indicated by analysis for water insoluble nitrogen (WIN) according to the method reported in Official Methods of Analysis of the Association of Official Agricultural Chemists, 11th Edition, 1970. WIN is used herein to denote quantitatively the amounts of insoluble ureaform solids. Some water soluble low molecular weight urea-formaldehyde methylene compounds are also ureaform compounds by virtue of chemical structure.
Ureaform is usually produced by the acid catalyzed reaction of urea and formaldehyde, or urea and urea-formaldehyde concentrates. The reaction is also catalyzed at a slower rate by the presence of acid or alkali ions even in neutral aqueous solutions. The ureaform reaction can continue until virtually all of the formaldehyde moieties are converted to high molecular weight water insoluble ureaform solids. Stopping the conversion of ureaform at a desired level of conversion to water insoluble solids, so that appreciable amounts of the ureaform compounds remain as readily available nitrogen nutrients for plants has been the object of much of the urea-formaldehyde fertilizer art. The further reaction of ureaform solids with urea and water soluble urea-formaldehyde compounds must be substantially stopped to produce a storage stable ureaform dispersion fertilizer. The ionic content of the dispersion aqueous liquids catalyzes this further reaction and makes the required stabilization very difficult. Heretofore the efforts made at stabilization have been to change the chemical structure of the ureaform solids.
Techniques which have been used include: drying to eliminate the catalytic effects of the ionic components of a ureaform solution; introduction of extraneous moieties such as sugars or higher aldehydes into the polymer chains; and caping the polymer chains with moieties which inhibit or stop further polymerization, such as ammonia, amines, or alcohols. Another technique used has been to prepare the insoluble ureaform in two stages; the first being carried out at relatively high temperatures and high formaldehyde to urea ratios to form methylene and methylene urea polymers; and the second being reaction with additional urea at lower temperatures.
Ureaform dispersion fertilizers offer an effective and safe method for supplying nutrients to turf and ornamental and commercial crops. A ureaform dispersion fertilizer may be defined as a flowable fertilizer containing urea-formaldehyde compounds and polymers, a substantial portion of which are water insoluble solid particles, dispersed throughout the fertilizer.
The production of a liquid ureaform fertilizer containing water insoluble nitrogen was disclosed in U.S. Pat. No. 3,677,736 by Richard E. Formaini. A liquid dispersion fertilizer containing ureaform having at least 1.5 percent WIN was produced by acidifying to a pH below 5, a dilute urea-formaldehyde reaction product, and heating to a temperature between 30.degree. and 80.degree. C., and neutralizing. Prior to acidifying, Formaini heated the urea-formaldehyde mixture under neutral, or basic, conditions until 60 percent of the formaldehyde was in the methylene form.
In U.S. Pat. No. 4,033,745, William P. Moore discloses the stabilization of ureaform fertilizer dispersions by utilization of alcohol and sugars as polymer caping moeities to stabilize the ureaform.
Paul Sartoretto et al disclosed in U.S. Pat. No. 4,298,512 aqueous dispersions of urea-aldehyde polymers for use as liquid fertilizers. The urea-aldehyde polymers are primarily ureaform, modified by the inclusion of one or more alkyl aldehydes containing from two to four carbon atoms. Even though Sartoretto recommends the use of urea-formaldehyde concentrate plus expensive and difficult to handle acetaldehyde or propionaldehyde, the reaction mixture is still quite exothermic and difficult to control. Urea-formaldehyde concentrate is a partially reacted mixture usually containing about 60 percent by weight formaldehyde, 25 percent urea, and the remainder water. Sartoretto states that when formaldehyde is reacted with urea the urea formaldehyde dispersions formed are unstable because polymerization and cross-linking apparently continue even at neutral pH, the solids becoming aggregated and lacking in dispersibility.
In U.S. Pat. No. 4,409,015 Thomas T. Grace discloses a two-stage process for preparing an aqueous dispersion fertilizer containing ureaform WIN, wherein he teaches reaction in the first stage of 2.5 to 4.7 mols formaldehyde with 1 mol of urea at about 90.degree. C. in the presence of an acid condensation catalyst until methylene and methyleneurea polymers have been formed of the desired length. These polymers are indicated by a special methanol solubility test indicating the intermediate to be storable for one year. Then, in the second stage the intermediate is reacted with an additional amount of urea by means of an acid catalyst until the amount of WIN is in the range of 15 to 35 percent of the total nitrogen of the system. Grace teaches substantially deactivating the formaldehyde in the first stage so that the reaction is less exothermic and easier to control in the second stage where the WIN is formed. The dispersion fertilizer product from the second stage has a preferred urea to formaldehyde mol ratio of 1.1 to 1.7 to 1. Grace further teaches the use of dicyandiamide to enhance the stability of the dispersion fertilizer by reacting with aldehyde groups present, presumably caping the WIN polymers.
Total nitrogen contents ranging between 15 and 18 percent by weight, and preferably 16 to 16.5 percent, are prescribed by Grace with the WIN preferably between 20 and 30 percent of the total nitrogen. No quantitative specification of, or reference to, unreacted urea contents are provided in U.S. Pat. No. 4,409,015. However, it is known that where methylene groups exist, particularly as polymethylene chains or methylene urea, the amount of additional urea which can be reacted is significantly lower than with non-polymerized urea and formaldehyde, or monomethylolurea. Liquid ureaform dispersion fertilizers are used commercially for turfgrasses, ornamental plants, and foliar feeding crops in areas which are long distances from locations of possible manufacture so that storage stability and concentration are important factors in the costs of shipping, and in the ultimate utility of the product. Maximum concentrations of previously disclosed liquid dispersion fertilizers are about 18 percent by weight. If one were to ship the highly storable first stage intermediate of U.S. Pat. No. 4,409,015, its shipping cost per unit of nitrogen would be high because its nitrogen content is only about 10 percent by weight.
It has long been an object of those skilled in the art to produce a ureaform dispersion fertilizer where the maximum amount of urea is protected per formaldehyde moiety used in a simple process generating little heat, from raw materials which are high in nitrogen concentrations and economical to ship and store.
It is therefore an object of this invention to provide a method of preparing a storage stable ureaform dispersion fertilizer containing economically significant amounts of water insoluble nitrogen (WIN).
It is a further object of this invention to provide an easily controlled method of reacting urea to ureaform dispersion fertilizers using minimum amounts of formaldehyde moieties.
It is another object of this invention to provide a ureaform dispersion fertilizer which does not solidify or settle during extended periods of storage.
It is still another object to provide a ureaform dispersion fertilizer which may be blended with other plant nutrients, such as phosphate and potash to form storage stable complete liquid fertilizers.